[spirv] Start the SPIR-V dialect
This CL sets up the basic structure for a SPIR-V dialect: operation
definition specification, dialect registration, testing, etc.
A single op, FMul, is defined and tested to showcase.
The SPIR-V dialect aims to be a simple proxy for the SPIR-V binary format
to enable straightforward and lightweight conversion from/to the binary
format. Ops in this dialect should stay as the same semantic level and
try to be a mechanical mapping to the corresponding SPIR-V instructions;
but they can deviate representationally to allow using MLIR mechanisms.
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PiperOrigin-RevId:
250040830